-----
Mike Hammett
Intelligent Computing Solutions
http://www.ics-il.com

Midwest-IX
http://www.midwest-ix.com

From: "Jay Hennigan" <jay@west.net>
To: nanog@nanog.org
Sent: Monday, October 10, 2022 10:39:06 AM
Subject: Re: any dangers of filtering every /24 on full internet table to preserve FIB space ?

On 10/10/22 07:58, Edvinas Kairys wrote:
> Hello,
>
> We're considering to buy some Cisco boxes - NCS-55A1-24H. That box has
> 24x100G, but only 2.2mln route (FIB) memory entries. In a near future it
> will be not enough - so we're thinking to deny all /24s to save the
> memory. What do you think about that approach - I know it could provide
> some misbehavior. But theoretically every filtered /24 could be routed
> via smaller prefix /23 /22 /21 or etc. But of course it could be a
> situation when denied /24 will not be covered by any smaller prefix.
>
> What do you think about this approach ?

Are you multi-homed? If not you can simply take a default.

If so, a better approach might be to apply a max AS rule and take full
tables plus a default from both (all). Something like "bgp maxas-limit
4" will optimize routing down to /24 but drop routes with long AS paths
and punt to default, reducing your table size at the cost of sub-optimal
routing to destinations that are going to take a convoluted path anyway.

--
Jay Hennigan - jay@west.net
Network Engineering - CCIE #7880
503 897-8550 - WB6RDV